A level indicator is registering a liquid level that is falsely low.
The operator has hand-gauged the storage vessel with a tape measure and determined the actual level to be 9 feet, but the level indicator (LI) registers 7.5 feet.
The calibrated range of the 4-20 mA transmitter is 0 feet to 12 feet.
You measure the current signal with your multimeter and find that it is 14 mA.
Which instrument is at fault in this system? How do you know?
Answer:
The transmitter is at fault, not the indicator.
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Credits: Tony R. Kuphaldt
4 comments
An indicator is usually connected in serial with the transmitter in the same circuit loop.
It`s not the indicator the measuring device and the fault is likely to be in the transmitter and not otherwise.
However, measuring the instrument`s output wherever in the loop is always the way to determine the right actual instrument`s output to the control system and frequently this is enough to solve the problem that we are dealing with but sometimes, STILL, it is not the guaranty that it will be processed in the right way by the control system (PLC or DCS) and displayed at DCS screen`s operator.
The transmitter is calibrated in wrong range, if it is calibrated in 0-12 Feet the output should be 16 mA instead 14 mA.
If the output is 14 mA with actual level is 9 feet then the LT calibration is 0-14..4 Feet.
In other side the LI calibrated in correct range 0-12 Feet.
Level Transmitter Should Be faulty … this could be on A sensor Primary element.
Why?
Each 3ft = 4mA
9 ft => 4+12 mA = 16 mA => LT is generating a wrong signal
14 mA => 7.5 ft => Li is rading Right
The LI is correctly outputing the level according to the amount of current received from the LT. A 14 ma current received from LT is correctly converted to 7.5 ft (heigh = 12ft*(14ma-4ma)/16)
On the other hand, at an actual level of 9ft, the LT was expected to output a 16ma current ( (16*9/12) + 4). 75%